--- a/testing/marionette/capabilities.js
+++ b/testing/marionette/capabilities.js
@@ -360,17 +360,17 @@ class Proxy {
 /** WebDriver session capabilities representation. */
 class Capabilities extends Map {
   /** @class */
   constructor() {
     super([
       // webdriver
       ["browserName", appinfo.name],
       ["browserVersion", appinfo.version],
-      ["platformName", Services.sysinfo.getProperty("name").toLowerCase()],
+      ["platformName", getWebDriverPlatformName()],
       ["platformVersion", Services.sysinfo.getProperty("version")],
       ["pageLoadStrategy", PageLoadStrategy.Normal],
       ["acceptInsecureCerts", false],
       ["timeouts", new Timeouts()],
       ["proxy", new Proxy()],
 
       // features
       ["rotatable", appinfo.name == "B2G"],
@@ -494,16 +494,33 @@ class Capabilities extends Map {
   }
 }
 
 this.Capabilities = Capabilities;
 this.PageLoadStrategy = PageLoadStrategy;
 this.Proxy = Proxy;
 this.Timeouts = Timeouts;
 
+function getWebDriverPlatformName() {
+  let name = Services.sysinfo.getProperty("name");
+
+  switch (name) {
+    case "Windows_NT":
+      return "windows";
+
+    case "Darwin":
+      return "mac";
+
+    case "Android":
+    case "Linux":
+    default:
+      return name.toLowerCase();
+  }
+}
+
 // Specialisation of |JSON.stringify| that produces JSON-safe object
 // literals, dropping empty objects and entries which values are undefined
 // or null.  Objects are allowed to produce their own JSON representations
 // by implementing a |toJSON| function.
 function marshal(obj) {
   let rv = Object.create(null);
 
   function* iter(mapOrObject) {

--- a/testing/marionette/test/unit/test_capabilities.js
+++ b/testing/marionette/test/unit/test_capabilities.js
@@ -366,16 +366,17 @@ add_test(function test_Proxy_fromJSON() 
   run_next_test();
 });
 
 add_test(function test_Capabilities_ctor() {
   let caps = new Capabilities();
   ok(caps.has("browserName"));
   ok(caps.has("browserVersion"));
   ok(caps.has("platformName"));
+  ok(["linux", "mac", "windows", "android"].includes(caps.get("platformName")));
   ok(caps.has("platformVersion"));
   equal(PageLoadStrategy.Normal, caps.get("pageLoadStrategy"));
   equal(false, caps.get("acceptInsecureCerts"));
   ok(caps.get("timeouts") instanceof Timeouts);
   ok(caps.get("proxy") instanceof Proxy);
 
   ok(caps.has("rotatable"));
 

--- a/testing/web-platform/meta/MANIFEST.json
+++ b/testing/web-platform/meta/MANIFEST.json
@@ -523104,17 +523104,17 @@
    "ab3c3d205e59df800ba5b4217245b83685521c31",
    "reftest"
   ],
   "css/css-scoping/shadow-host-with-before-after.html": [
    "99af6e29e69b3131b59dbdc2b0eead52931123c2",
    "reftest"
   ],
   "css/css-scoping/shadow-link-rel-stylesheet-no-style-leak.html": [
-   "76a54dabd8bd09f7155ab0331e3d17d1a0cae243",
+   "a46be006762a16c2deb3d1d3a760e3c4e348668c",
    "reftest"
   ],
   "css/css-scoping/shadow-link-rel-stylesheet.html": [
    "07862ce7d2a954988bdbce882869a4c5f097089a",
    "reftest"
   ],
   "css/css-scoping/shadow-reassign-dynamic-001.html": [
    "11ed4da2e6ce88d8a2b98a8f1c814417ef7770dd",
@@ -548596,17 +548596,17 @@
    "cd3f0233cc0eaf9295e602ca25aef87fb68df851",
    "support"
   ],
   "css/mediaqueries/support/min-width-tables-001-iframe.html": [
    "29e7fb34c94e2e8411514d1e71d09aca2ddb642e",
    "support"
   ],
   "css/mediaqueries/test_media_queries.html": [
-   "a7d78b13e119f8cd1ffa8812a9af67e59280084d",
+   "cff3585932589f611a7101329d3b5b6ca27820aa",
    "testharness"
   ],
   "css/mediaqueries/viewport-script-dynamic-ref.html": [
    "7d55c513e2de39c9b362fc864233a3008ca6ced2",
    "support"
   ],
   "css/mediaqueries/viewport-script-dynamic.html": [
    "1c2ba1a9116942599804ed29553e85628afadb04",
@@ -559700,17 +559700,17 @@
    "d1661ab1734f7d1a252030aeac7e9842a7a4cb3b",
    "testharness"
   ],
   "custom-elements/Document-createElement-svg.svg": [
    "9af8f2dc7778feeea4fa8e557d7885b10d325dea",
    "testharness"
   ],
   "custom-elements/Document-createElement.html": [
-   "46e64c9f412fb04582f8ec287e08783ac83cb933",
+   "0edab8da0f16d5d2239ffb21f446c371fe4c76c3",
    "testharness"
   ],
   "custom-elements/Document-createElementNS.html": [
    "da90b2a1c13cf18fd5cade85dcae2dadef6243c9",
    "testharness"
   ],
   "custom-elements/HTMLElement-constructor.html": [
    "4dc04a8b026538bddee52586f2df50206abc9334",
@@ -577844,17 +577844,17 @@
    "da39a3ee5e6b4b0d3255bfef95601890afd80709",
    "support"
   ],
   "html/resources/common.js": [
    "0f18ee2c61b99893cfe2a3d1ff549b170a8d715d",
    "support"
   ],
   "html/scripting/the-noscript-element/non-html-noscript.html": [
-   "c0c5453111f29e5a0206f988f4d127ec8ebc2f13",
+   "121760184777008c2ddeb598278216e40b34e367",
    "testharness"
   ],
   "html/semantics/.gitkeep": [
    "da39a3ee5e6b4b0d3255bfef95601890afd80709",
    "support"
   ],
   "html/semantics/OWNERS": [
    "abd95839027a88741c4d351ff374d81b773c80fa",
@@ -619080,41 +619080,41 @@
    "6274392529489518207961ed5e6b6bb3ff081572",
    "wdspec"
   ],
   "webdriver/tests/new_session/__init__.py": [
    "da39a3ee5e6b4b0d3255bfef95601890afd80709",
    "support"
   ],
   "webdriver/tests/new_session/conftest.py": [
-   "d2df38e506cb9a3e501f03fe03e2a31af42d6f04",
+   "93d472d4653fb54dabad9917b764966009add676",
    "support"
   ],
   "webdriver/tests/new_session/create_alwaysMatch.py": [
-   "84f8ad0908e9ec787f9db73ff66cbe193c2c9626",
+   "cf23ac1d3546917d4da9b78384e747356a1ca83f",
    "wdspec"
   ],
   "webdriver/tests/new_session/create_firstMatch.py": [
-   "fbdabf1852c07db0f5df2d47954c1cd5d7285492",
+   "c3f825382c246d26af2adf6911f522ea7cfa43e5",
    "wdspec"
   ],
   "webdriver/tests/new_session/default_values.py": [
    "92c26366f8fd0d37587457b9b5a630df4b19cc60",
    "wdspec"
   ],
   "webdriver/tests/new_session/invalid_capabilities.py": [
-   "fcc36e5ca46cb8a9fb9927d5f618c7e9433a0260",
+   "440bcb7b7b5f316c9b239afc40f99e3deb94c759",
    "wdspec"
   ],
   "webdriver/tests/new_session/merge.py": [
-   "000b28631bb6d862b654f1f998852f5f1a6b2e12",
+   "856d7c846766b3dd2bc6f23026aa23910407e23d",
    "wdspec"
   ],
   "webdriver/tests/new_session/response.py": [
-   "5eabb99858b9c3051efb351096d7ba8dca8dbfce",
+   "3be8cfd44eb4c9908ec65e66dcdc6a9ea0e5241b",
    "wdspec"
   ],
   "webdriver/tests/new_session/support/__init__.py": [
    "da39a3ee5e6b4b0d3255bfef95601890afd80709",
    "support"
   ],
   "webdriver/tests/new_session/support/create.py": [
    "4bfb99fd4ba44ab5b4e70e9246cbb200c098ae89",

--- a/testing/web-platform/tests/webdriver/tests/new_session/conftest.py
+++ b/testing/web-platform/tests/webdriver/tests/new_session/conftest.py
@@ -6,17 +6,17 @@ import webdriver
 
 def product(a, b):
     return [(a, item) for item in b]
 
 
 def flatten(l):
     return [item for x in l for item in x]
 
+
 @pytest.fixture(scope="session")
 def platform_name():
     return {
         "linux2": "linux",
         "win32": "windows",
         "cygwin": "windows",
         "darwin": "mac"
     }.get(sys.platform)
-

--- a/testing/web-platform/tests/webdriver/tests/new_session/create_alwaysMatch.py
+++ b/testing/web-platform/tests/webdriver/tests/new_session/create_alwaysMatch.py
@@ -1,13 +1,12 @@
-#META: timeout=long
+# META: timeout=long
 
 import pytest
 
 from conftest import product, flatten
 
 from support.create import valid_data
 
 
 @pytest.mark.parametrize("key,value", flatten(product(*item) for item in valid_data))
 def test_valid(new_session, add_browser_capabilites, key, value):
     resp = new_session({"capabilities": {"alwaysMatch": add_browser_capabilites({key: value})}})
-

--- a/testing/web-platform/tests/webdriver/tests/new_session/create_firstMatch.py
+++ b/testing/web-platform/tests/webdriver/tests/new_session/create_firstMatch.py
@@ -1,9 +1,9 @@
-#META: timeout=long
+# META: timeout=long
 
 import pytest
 
 from conftest import product, flatten
 
 from support.create import valid_data
 
 

--- a/testing/web-platform/tests/webdriver/tests/new_session/invalid_capabilities.py
+++ b/testing/web-platform/tests/webdriver/tests/new_session/invalid_capabilities.py
@@ -1,9 +1,9 @@
-#META: timeout=long
+# META: timeout=long
 
 import pytest
 from webdriver import error
 
 from conftest import product, flatten
 
 
 @pytest.mark.parametrize("value", [None, 1, "{}", []])
@@ -90,9 +90,8 @@ invalid_extensions = [
 def test_invalid_extensions(new_session, add_browser_capabilites, body, key):
     capabilities = body(key, {})
     if "alwaysMatch" in capabilities:
         capabilities["alwaysMatch"] = add_browser_capabilites(capabilities["alwaysMatch"])
     else:
         capabilities["firstMatch"][0] = add_browser_capabilites(capabilities["firstMatch"][0])
     with pytest.raises(error.InvalidArgumentException):
         resp = new_session({"capabilities": capabilities})
-

--- a/testing/web-platform/tests/webdriver/tests/new_session/merge.py
+++ b/testing/web-platform/tests/webdriver/tests/new_session/merge.py
@@ -1,9 +1,9 @@
-#META: timeout=long
+# META: timeout=long
 
 import pytest
 from webdriver import error
 
 from conftest import platform_name
 
 
 @pytest.mark.skipif(platform_name() is None, reason="Unsupported platform")
@@ -72,10 +72,10 @@ def test_merge_browserName(new_session, 
                                 "pageLoadStrategy": "none"
                             },
                             {
                                 "browserName": browser_settings["browserName"],
                                 "pageLoadStrategy": "eager"
                             }
                         ]}})
 
-    assert resp["capabilities"]["browserName"] == browser_settings['browserName']
+    assert resp["capabilities"]["browserName"] == browser_settings["browserName"]
     assert resp["capabilities"]["pageLoadStrategy"] == "eager"

--- a/testing/web-platform/tests/webdriver/tests/new_session/response.py
+++ b/testing/web-platform/tests/webdriver/tests/new_session/response.py
@@ -1,12 +1,13 @@
 # META: timeout=long
 
 import uuid
 
+
 def test_resp_sessionid(new_session, add_browser_capabilites):
     resp, _ = new_session({"capabilities": {"alwaysMatch": add_browser_capabilites({})}})
     assert isinstance(resp["sessionId"], unicode)
     uuid.UUID(hex=resp["sessionId"])
 
 
 def test_resp_capabilites(new_session, add_browser_capabilites):
     resp, _ = new_session({"capabilities": {"alwaysMatch": add_browser_capabilites({})}})
@@ -44,11 +45,12 @@ def test_resp_data(new_session, add_brow
 def test_timeouts(new_session, add_browser_capabilites, platform_name):
     resp, _ = new_session({"capabilities": {"alwaysMatch": add_browser_capabilites({"timeouts": {"implicit": 1000}})}})
     assert resp["capabilities"]["timeouts"] == {
         "implicit": 1000,
         "pageLoad": 300000,
         "script": 30000
     }
 
+
 def test_pageLoadStrategy(new_session, add_browser_capabilites, platform_name):
     resp, _ = new_session({"capabilities": {"alwaysMatch": add_browser_capabilites({"pageLoadStrategy": "eager"})}})
     assert resp["capabilities"]["pageLoadStrategy"] == "eager"